BPy Image_Editor - remove Image completely problem
UI redraw issue.

The issue you are seeing is UI related. The red is an indicator of this: warning that there is an issue, in this case showing the name of an image that has been removed. 

If this is the case then issuing a tag redraw to the image editor will fix .

import bpy

img = bpy.data.images[0]
bpy.data.images.remove(img)

for a in bpy.context.screen.areas:
    if a.type == 'IMAGE_EDITOR':
        a.tag_redraw()